Brushless direct current motor system

ABSTRACT

The invention relates to a brushless direct current motor system that includes an outer rotor receiving a load member such as a fan wheel having a plurality of blades. An axial extension of the rotor functions as a drive boss whose diameter is independent of the diameter of the motor stator core and is step-wise smaller in diameter at its outer closed end than at its inner opened end.

This is a continuation, of application Ser. No. 581,337 filed Feb. 17,1984 now U.S. Pat. No. 4,554,473, issued Nov. 19, 1985 which is acontinuation-in-part of application Ser. No. 244,971, filed Mar. 18,1981 now abandoned.

The invention relates to a brushless direct current motor system.

Brushless direct current motors are typically provided with an electriccircuit which forms the drive electronic systems in the instance of diskstorage drives, and r.p.m. control stage for the motor. The driveelectronic system comprises the electronic components that replacebrushes and commutators of conventional direct current motors. Ther.p.m. control may include an optical tachometer, which senses ther.p.m. of the motor shaft and emits corresponding signals to theelectronic drive system to control the r.p.m. of the shaft. At the sametime the optical unit of the tachometer can be expediently applied asposition-determining means for determining the position of the rotor inorder to switch the currents of the motor windings. Instead of thetachometer as a position detector, a Hall-generator can be also used ora Hall-generator in conjunction with an IC.

The electronic drive system and the r.p.m. control circuit sometimes areprovided in a control unit separate from the motor. Such a separatecontrol unit takes additional space and requires a cable for connectingthe control unit with the motor.

In some uses, a separate control unit of conventional direct currentmotors is inconvenient because of limited space. For example, magneticdisk or plate drive systems are desirably as small and compact aspossible, yet the drives of the magnetic disks should desirably lendthemselves to the same possible uses as for larger magnetic plate drivesystems. Brushless direct current motors are particularly adapted forsuch drives because of their reliability, absence of sparks, which occurwith brushes of conventional direct current motor, and simple designwhen such a motor has a part which doubles as a boss for supporting, forexample, two spaced apart magnetic plates. Conventional brushless directcurrent motors, however, have been unsatisfactory in such instancebecause of inadequate space within the motor for the associated controlunit.

Brushless direct current motors also are used in axial-flow fans, butmany axial-flow fans are installed in electronic appliances to serve asventilators where there is a size restriction. Thus, the externaldimensions of the casing jacket are likely predetermined and fixed. Thisbecomes critical in very small ventilators, because of the proportion ofthe diameter of the motor to the diameter of the surrounding fanhousing. Because of this, there is a limit to the amount of radialdimension available for the fan blades. It is important, therefore, thatthe air input be as efficient as possible to maximize the air deliveryof the fan even in spite of static pressures often encountered in faninstallations. This invention is advantageously useful for brushlessdirect current motors used in both axial-flow fans and in disk storagedrives.

An object of the invention is to provide a brushless direct currentmotor system having a rotor of simple design as a boss for a stack ofdisks, particularly small diameter disks driven by a high qualitybearing system.

Another object of the invention is to provide a brushless direct currentmotor system having a rotor of simple design usable as a boss forcarrying and driving disks or as a fan wheel for a plurality of fanblades directly attached or welded to the boss, the boss being driven bya high quality bearing system.

Still another object of the invention is to provide a brushless directcurrent motor of the outer rotor type that includes a step-wise smallerdiameter on the closed end of the outer rotor than the diameter at theopen end thereof such that for use in driving disks, disks ofstandardized bores may be driven and for use in axial flow fans, theinlet side may have a better flow structure to permit higher volumes ofair or at least permit a press fit of a fan hub over the closed end ofthe housing without increasing the overall diameter of the rotorhousing.

By providing a brushless direct current motor system having an outerrotor, and having at least one stationary switch support supportinglymounted on the motor stator these objects are solved, according to theinvention, by mounting the electronic drive system on the stator withinthe closed end of the outer rotor housing.

Thus, according to the invention, the electronic drive system canconstitute a part of the motor in a brushless direct current motorsystem. An axially compact structural unit is provided which lendsitself to many applications. It will be seen that the structure allows alarger axial distance between bearings to provide a strong bearingsystem. According to a preferred embodiment, the drive shaft issurrounded by means which hold the bearings that in turn support therotatable motor shaft. The means is disposed in the motor housing. Acircuit component board is connected to the means and is preferablyconstructed as an annular, plate-shaped member to support one or bothsides of at least part of the electronic drive system. The dimensions ofthe annular component board are expediently adapted to the motorhousing. Also a plurality of such members can be disposed one overanother in accordance with the height of the available axial space. Insuch a way the motor system may provide a closed unit with only leadlines extending outwardly for connection to an outside source of power.In operation, the space-saving overall arrangement is reliable. Thesystem also includes means for dissipating heat and it allows astep-wise construction on the hub side of the motor, advantageous foruses of the motor in both disk drives and axial flow fans.

The invention will now be described in greater detail showingembodiments illustrated in the accompanying drawings, which are:

FIG. 1 is a side sectional view of a brushless direct current motorsystem according to the invention;

FIG. 2 is a side sectional view, taken along the line A--A of FIG. 5, ofanother embodiment utilizing the invention in an axial-flow fan whereradial blades are attached directly to the outer rotor and the housingis asymmetrical in section;

FIG. 3 is a side sectional view, taken along the line A--A of FIG. 5, ofyet another embodiment utilizing the invention in an axial-flow fanwhere a fan wheel is inserted with its hub press-fit over the outerrotor and the housing is asymmetrical in section;

FIG. 4 is a side sectional view, taken along the line A--A of FIG. 5 ofstill a third embodiment utilizing the invention in an axial-flow fan ina manner similar to that shown in FIG. 3, except that the housing issymmetrical in section; and

FIG. 5 is a schematic view of an axial-flow fan according to theinvention as seen from the exhaust side.

Referring first to FIG. 1, the direct current motor system 10 comprisesa brushless direct current motor 11 with an outer rotor .Iadd.housing.Iaddend.14 which, being mounted on a rotatable drive shaft 12,surrounds motor windings 29, stator .Iadd.iron core .Iaddend.58 andpermanent magnets 56.

The foregoing components of motor 11 are known as such. Their assemblyconstitutes a conventional brushless direct current motor.

An annular switch support or component board 20 is connected to astationary part or a central support or holding means 22 of the motor11. An electronic drive system and an r.p.m. control circuit are mountedon the component board 20 to form together with the motor 11 a compactunit.

As shown in FIG. 1, the various components of the electronic drivesystem, for example, output stage transistors 61 of the r.p.m. controlcircuit, a Hall-IC 35, the component board 20 and a heat sink 60, areaccommodated in one chamber between the bottom or closed end 40 of therotor housing 14 and the stator winding 29. Lead lines 31 extend fromthe component board 20 for connection to a source of direct voltage.

The shaft 12 is attached to the .[.bottom.]. .Iadd.closed end.Iaddend.40 of housing 14. The housing 14 is open at one end so that acentrally disposed bearing tube or casing 44 surrounding the shaft 12can extend from the housing. The tube 44 is a part of central holdingmeans 22. It is either pressed into a boss connected to a flange 30 oris cast together with the flange 30 to form an integral part. The shaft12 is rotatably supported in the tube or casing 44 in two bearings 48,48', so that the shaft and the housing 14 rotate relative to the casingand the flange 30 when the flange is mounted in a stationary positionrelative to adjacent members, as will be explained hereinafter indetail. Two spaced guard rings 50 maintain the bearings in place. Cupsprings 52 held by the guard ring 54 axially bias the bearings 48, 48'relative to each other.

A rotary magnetic ring or a plurality of permanent magnets 56 aremounted on the inside surface of the rotor housing 14, and they rotatetogether with the housing 14. The rotor housing 14 may be cast of analuminum alloy. This material is unsuitable as a magnetic returncircuit, and so a soft iron ring 57 is provided for the magnetic returncircuit. Such ring is unnecessary if the housing 14 is deep-drawn. Theouter diameter of the annular component board 20 is preferably less thanthe diameter of the core 58. Thus, the component board 20 is not greaterin diameter than the air gap diameter of the stator and, togethertherewith, makes a compact unit.

The bearing tube 44 is axially extended for receiving the electronicmembers of the electronic drive system and r.p.m. control circuit. Inaddition to the disposition of the structural members, care must be alsotaken of suitable dissipation of heat, particularly in connection withthe output stage transistors 61 of the electronic drive system. There isprovided for such purpose a punched bent member as a cooling body orheat sink 60, to which are connected in a heat-transfer relationship,the heat dissipation surfaces of the output stage transistors 61. Theheat sink 60 itself is about 0.6 to 1 mm thick, connected by supports 62to the component board 20.

The electronic members are expediently arranged as shown in FIG. 1,since all soldering connections can there be performed in one step, as,for example, by dip soldering. A potentiometer 64 is provided foradjusting certain portions of motor operation or for equalizingtolerances of different parts. The potentiometer 64 is adjustable by ascrewdriver through a bore (not illustrated) in the flange 30 and one ofthe grooves in the core 58. For such purpose, the setscrew of thepotentiometer 64, the bore in the flange 30 and the groove in the.Iadd.stator iron .Iaddend.core 58 lie in a substantially straight line.

In such an integral formed part as the heat sink 60, the output stagetransistors 61 must be electrically insulated from, although physicallyconnected to, the heat sink 60. If the heat sink 60 is expedientlysubdivided according to the number of the output stage transistors 61,however, the insulation can be dispensed with. This saves time ofassembly and various insulating parts and also improves the dissipationof heat.

The heat sink or sinks 60 are connected to the component board 20,preferably via the supports 62 soldered to the component board 20. Theheat sinks 60 are soldered to the component board 20 along with thesoldering of the other members. If necessary, the heat sinks 60 areadditionally connected to the stationary parts, or stator in a suitablemanner, such as by fastening members or linking connections 66 on thetube 44 or in the zone of a collar 68 or by mounting legs of the endplate 67 .[.(not illustrated.].. These connections reduce thepossibility of the movement of the heat sink 60.

The rotor housing 14 is axially extended in accordance with .[.the.].axial extension of bearing tube 44. This extension projects into .[.thedrive boss.]. .Iadd.a drive boss or hub .Iaddend.70 in the central boreof one or more magnetic disks. Consequently, it is expedient to make thediameter of the drive boss 70 complementary to or substantially equal tothat of the central bore of the disks regardless of the required drivingcapacity of the motor and of the resulting best air gap diameter. Thus,the axial dimension is lengthened by a strong bearing system to allowthe .[.hub.]. .Iadd.drive boss .Iaddend.70 to have a diameter that isindependent of the diameter of the stator core 58 and the air gap. Thispermits a free chamber 26 inside the drive boss 70 to accommodate thecomponent board, including the electronic members and the heat sinks. Asmentioned earlier, the component board is preferably of less diameterthan the stator iron and air gap.

In a drive motor for magnetic memory disks, the housing 14, includingthe drive boss 70 and the bottom 40, encloses the inside of the motor.This prevents the penetration of dirt particles from inside the motor tothe magnetic disks. The only direct connection to air inside the motorexists only through a gap 71, between the rotor .Iadd.housing.Iaddend.14 and the flange 30. But a suitable length of the gap 71provides that the possible exit of dirt particles is very small andthere is no disturbance in the safety of operation of the magneticdisks. A wall 72 is shown as a broken fragment and separates a space ofmaximum cleanness 73 from the rest of the motor system so that any dirtparticles from the bearing 48' do not enter the area of the magneticdisks.

When used for driving magnetic disks, the motor 11 includes a fan 32having several fan blades 33 mounted at the free end of the drive shaft12. The flange 30 is cooled by an intensive movement of air around it,so that lost heat is effectively conducted from the motor 11 to theoutside via integral unit which includes the flange 30 and the bearingtube 44 as mentioned earlier.

To prevent an electronic charge in the rotor bell, the drive shaft 12is, by means of the ball 36 and a spring contact (not illustrated),connected to the chassis of the apparatus. An electrical charge of therotor would likewise disturb the operation of the magnetic disks.

The motor 11 is also suitable for other applications. In such cases therequirement of cleanness is not as exacting, so that there also can beapertures 59 in the rotor bell, if necessary. These apertures wouldperform the function of evacuating heat or of permitting access foradjusting a potentiometer. Such motors can directly drive differentdevices or fans. For the purposes of ventilation, fan wheels or fanblades can be directly attached or welded to the rotor housing 14 and/orthe drive boss 70.

In this connection, it should be noted that the same features of thisinvention that permit the .[.hub.]. .Iadd.drive boss .Iaddend.70 to bereduced in diameter so as to accept directly the magnetic disks havingstandardized bores, also make the motor 11 advantageously useful foraxial-flow fans. As disclosed in copending U.S. Ser. No. 140,883, filedApr. 16, 1980, .Iadd.now U.S. Pat. No. 4,373,861, issued Feb. 15, 1983,.Iaddend.and in copending U.S. Ser. No. 466,642, filed Feb. 15, 1983,.Iadd.which through continuation application Ser. No. 718,832, filedApr. 2, 1985, became U.S. Pat. No. 4,564,335, issued Jan. 14, 1986,.Iaddend.the feature of a conical configuration on the hub of the fanwheel on the inlet side in cooperation with the flaring corner portionsor pockets of the fan housing, surprisingly produces the advantage of asubstantial pressure increase. Thus, particularly in the instance ofsmall, compact axial-flow fans operating in the region of relatively lowair flow rates, the fan can deliver a larger amount of air at higherstatic pressures.

Such a construction can be seen in connection with motor 11a in FIG. 2wherein an indented or stepped outer portion of the .[.hub.]..Iadd.drive boss .Iaddend.70 provides a conical or tapered affect asillustrated by the dashed line L. In FIGS. 2, 3 and 4, like parts ofthose motors and the motor of FIG. 1 carry the same reference numbers.The direction of the air flow is indicated by the arrow. .Iadd.Thus airenters these axial-flow fans at an inlet or input side 13 and isdischarged from an outlet or exhaust side 15. .Iaddend.A plurality offan blades 23 are uniformly distributed around the periphery of therotor housing 14 and are suitably affixed to the outer surface of therotor housing such as by welding. These fan blades extend oversubstantially the entire axial length of the rotor housing 14. Thus, atthe air inlet side of the fan, it will be seen from an examination ofthe angle of the dashed line L that the smaller diameter of the.[.hub.]. .Iadd.drive boss .Iaddend.70 provides a greater cross sectionof air passage on the inlet side than would be possible if the diameterof the .[.hub.]. .Iadd.drive boss .Iaddend.70 were equal to the outerdiameter of the rotor housing 14 at its open end.

As used herein in connection with the cup-shaped rotor .Iadd.housing.Iaddend.14, the terms "open end" and "closed end" are used mainly todistinguish between the two ends. Thus the term "closed", particularlyin reference to a motor used in fans, need not exclude small openingsthat may be provided in the otherwise closed end of the rotor housing.

Each of the motors shown in FIGS. 2, 3 and 4 have a fan housing 17 thatforms the outer extremes and provides structure for mounting the motorand fan wheel or fan blades. The housing 17 dimensions are predeterminedaccording to the standards in the industry, and the housing 17 may havesmall dimensions where the square outside faces have a length of about80 mm. The fan housing 17 includes a central cylindrical portionindicated by 17b, and eminating from that toward the inlet side is aninclined wall section 17a. The inclined wall section 17a is provided onat least four diagonally opposite corners of an externally square fanhousing 17.

Returning to FIG. 2, it can be seen that by means of this arrangement abroader inlet cross section is obtained over the entire inlet zone ofthe axial-flow fan. The front edges of the fan blades 23 extend intothis cross section, and they are brought up approximately to the forwardend face of the fan housing 17 with their leading edges. At their inneredges, the fan blades are welded to the rotor housing 14, particularlyat the drive boss 70. The fan blades 23 may be welded conventionally tothe rotor housing 14 by a special process disclosed in German Pat.1,628,349. The fan blades are made of very thin sheet steel, and arearranged equally around the periphery, and a very large free inletregion is obtained which is enlarged even more by the provision betweenthe angled dashed line L and the inclined .[.wall 17a.]. .Iadd.wallsection 17a.Iaddend..

Because of the fan housing 17, as can be seen from .Iadd.a combinationof .Iaddend.FIGS. 2 and 5, is provided, on the four diagonally oppositecorners of the externally square fan housing 17, with corner pockets 18,the inclined wall sections 17a of which eminate from the centralcylindrical .[.middle piece.]. .Iadd.portion .Iaddend.17b, the inletcross section is additionally enlarged at these four corners. It hasbeen found that an axial-flow fan constructed in this way has advantagesover conventional kinds of construction, especially if it must operateagainst high static pressures in the installed condition. In suchinstance, the fan yields surprisingly larger amounts of air.

The welding process for attaching the fan blades 23 to the rotor housing14 as described in the aforementioned German patent, is useful for massproduction and generally requires a very large, expensive weldingmachine. If axial-flow fans are to be produced in smaller quantitites,or if they are to be produced in locations far distant from the weldingmachine, it is much less expensive to produce a fan wheel of moldedplastic. There, the blades distributed evenly about the periphery of thehub, are molded along with the hub to form a unit.

The invention of this application, however, also lends itself to use inaxial-flow fans having the unitized molded plastic fan wheel. Such amotor is shown in FIG. 3, and the molded plastic fan wheel 19 having ahub 19a which carries the evenly distributed fan blades 23a about itsperiphery, is applied to the rotor housing 14 by being press-fit overthe .[.reduced hub portion.]. .Iadd.reduced portion of drive boss.Iaddend.70 at the rotor housing 14 and secured in a suitable manner.Comparing this structure with that in FIG. 2, it will be seen that theangled dashed line L, if drawn in the structure of FIG. 3, would besubstantially vertical. That is, the outer diameter of the plastic hub19a is substantially the same as the outer diameter of the rotor housing14 near its open end.

The advantage of the plastic fan wheel, of course, is that it results inan axial-fan of overall less expense than that shown in FIG. 2. It canbe understood, however, that the outer diameter of the hub 19a is stillof a smaller dimension than had it been placed over the outer rotor of aconventional DC brushless motor. Thus, this invention finds advantageoususe in this application, also.

The motor structure shown in FIG. 4 is similar to that in FIG. 3 exceptfor one important difference. It will be noted in FIG. 3 that thecentral cylindrical .[.mid piece.]. .Iadd.portion .Iaddend.17b isflanked by an inclined .[.wall 17a.]. .Iadd.wall section 17a.Iaddend.toward the inlet side and an .[.inclined wall.]. .Iadd.inclinedwall section .Iaddend.17c toward the outlet side. By inspection, it willbe seen that the .[.wall 17a.]. .Iadd.inclined wall section 17a.Iaddend.is shorter and at a slightly larger angle to the.[.mid-piece.]. .Iadd.central cylindrical portion 176 .Iaddend.than thewall 17c. Such a fan housing is referred to as an asymmetrical housing.In contrast, the structure in FIG. 4 is a symmetrical housing where theangle and length of the .[.walls.]. .Iadd.inclined wall sections.Iaddend.17a and 17c flanking the central cylindrical .[.mid-wall.]..Iadd.portion .Iaddend.17b are substantially the same length and of thesame angle with respect to the .[.mid piece wall.]. .Iadd.centralcylindrical portion.Iaddend.. The reduced diameter .[.hub.]. .Iadd.driveboss .Iaddend.70 finds advantageous use in both of these structures.

It has been indicated that the bearing system in a brushless DC motorfor direct drive of magnetic disks must be of high quality and accuracy.Hence, the motor shown in FIG. 1, in which the load on the .[.hub.]..Iadd.drive boss .Iaddend.70 is a plurality of magnetic disks, is shownwith spaced apart ball bearings 48 and 48'. These bearings are heldsecurely within the bearing tube 44 which constitutes the centralholding means 22. Not all requirements for brushless DC motors, however,are as rigid as those for direct drive of magnetic disks. In thisconnection, it will be noted in the axial fans shown of FIGS. 2, 3 and4, that spaced apart ball bearings are shown in the motor 11a in FIG. 2,whereas sleeve bearings 49 are shown in motors 11b and 11c in FIGS. 3and 4, respectively.

Although the central holding means 22 is shown to be the bearing tube 44in the drawings, it should be understood that for many applications, itwould be acceptable for the central holding means to consist merely ofthe inside of the stator iron core 58. Thus, the stator iron couldprovide the support for either the ball bearings 48, 48' or the sleevebearings 49 in certain applications of the DC brushless motor. Thecomponent board 20 would then be supported by means of pins to thestator in a suitable location.

A further modification of the structure is in providing the motors 11band 11c of FIGS. 3 and 4, respectively, with the fan housing 17, thecentral holding means 22, the flange 30a, and the supporting struts 24(FIG. 5) as a one-piece molded plastic. At present, such structure haslimited applications and is thought to be useful only in the instance ofa two-pulse brushless DC motor, especially a reluctance torque motorhaving auxiliary torques, all resulting in high efficiency and low heatgeneration. Heretofore, plastic has been used only in the veryinexpensive installations, because heat is destructive to the rigidityof the plastic structure. With the more highly efficient DC brushlessmotors with low heat generation, however, a plastic structure is nowpractical for certain other applications.

Thus, there has been provided in accordance with this invention aninternal motor structure for a brushless direct current motor systemincluding the electronic drive system and r.p.m. control circuit mountedon a component board internally of the motor in such a manner as toenable a step-wise smaller diameter on the closed end of the hub of theouter rotor than the diameter of the open end of the outer rotor. Suchstep-wise smaller diameter allows for such a motor to be used in thedirect drive of magnetic disks, where the disks have standardized bores,and in axial-fans where a larger cross section on the air input side ofthe fan, particularly a fan of small dimensions, to be used inapplications where it is important to deliver higher volumes of air athigher pressures. Some principal advantages of such a motor structureare (1) axial compactness, (2) easy access to the electronic circuitboard during motor production and repair, and (3) independency of thediameter of the drive boss to the stator and air gap.

Although we have shown and described this invention in connection withcertain embodiments, alternatives, modifications, and variations may beapparent to those skilled in the art in view of the foregoingdescription. Accordingly, it is intended to embrace all suchalternatives, modifications, and variations as fall within the spiritand scope of the appended claims.

What is claimed is:
 1. A brushless direct current motor system, saidsystem including an outer rotor type motor comprising:a cup-shaped.Iadd.outer .Iaddend.rotor housing having .Iadd.inner and outersurfaces, an open and a closed end and .Iaddend.a shaft concentricallymounted therein extending internally thereof, the outer .[.portion.]..Iadd.surface .Iaddend.of said housing being formed as a drive boss toreceive .Iadd.and directly drive .Iaddend.at least one load member.[.for the direct driving thereof.]., the load member being a fan wheelhaving a plurality of blades, the fan wheel being a unitized assemblyincluding a hub and radially extending fan blades mounted thereonuniformly .[.around the periphery.]. .Iadd.therearound .Iaddend.; abearing system in said motor receiving said shaft so as to rotate said.Iadd.outer .Iaddend.rotor housing; stator means concentrically mountingsaid bearing system within said motor, said .Iadd.outer .Iaddend.rotorhousing including permanent magnets on its .[.inside.]. .Iadd.inner.Iaddend.surface and being mounted to rotate around .[.the peripheryof.]. said stator means .[.and.]. separated .Iadd.therefrom .Iaddend.byan air gap, the .[.outside.]. .Iadd.outer surface .Iaddend.of said.Iadd.outer .Iaddend.rotor housing at its closed end being provided witha step-wise reduction in its outer diameter to provide a reduceddiameter hub portion .[.thereof, allowing the outside of this portion ofthe rotor housing.]. to mount and drive said unitized assembly; and thehub of said unitized assembly being .[.dimensioned to be press fitover.]. .Iadd.fixed to .Iaddend.the reduced diameter hub portion of saidouter rotor .Iadd.housing.Iaddend..
 2. A system according to claim 1wherein the diameter of the hub of said fan wheel is not greater thanthe diameter of the cup-shaped .Iadd.outer .Iaddend.rotor housing at itsopen end.
 3. A system according to claim 2 wherein said system is partof an axial-flow fan .Iadd.having an air inlet side and an air exhaustside .Iaddend.and further comprising an outer fan housing ofmulti-corner profile, said .Iadd.outer fan .Iaddend.housing having acentral cylindrical portion surrounding the fan.[.blade.]..Iadd.wheel.Iaddend., said .Iadd.outer fan .Iaddend.housingbeing broadened on the exhaust side of the .Iadd.central.Iaddend.cylindrical .[.central.]. portion by corner pockets into the.[.regular.]. multi-corner profile, said profile circumscribing .[.theouter diameter of.]. the fan wheel.
 4. A system according to claim 3wherein said corner pockets are formed by inclined .[.walls.]..Iadd.wall sections .Iaddend.which extend from said central cylindricalportion.
 5. A system according to claim 4 wherein the inclined wall.Iadd.sections also extend from the central cylindrical portion.Iaddend.on the air inlet side .[.is.]. .Iadd.and are .Iaddend.axiallyshorter than and at a different angle from the inclined wall.Iadd.sections .Iaddend.on the .[.outlet.]. .Iadd.exhaust .Iaddend.sideof the central cylindrical portion, forming an asymmetrical crosssection in the .Iadd.outer .Iaddend.fan housing.
 6. A system accordingto claim 4 wherein the inclined .[.walls.]. .Iadd.wall sections alsoextend from the central cylindrical portion and these sections.Iaddend.on both the inlet and .[.outlet.]. .Iadd.exhaust .Iaddend.sidesof the central cylindrical portion are of substantially .[.the.]..Iadd.a .Iaddend.same angle and length .Iadd.with respect .Iaddend.toeach other, forming a symmetrical cross section in the .Iadd.outer.Iaddend.fan housing.